Tuning control system



June 20, 1950 R K lNN 2,512,336

TUNING CONTROL SYSTEM Filed Oct. 12, 1945 INVENTOR Theodore P.Kinn.

ATTORNEY Patented June 20, 1950 UNITED STATES PATENT OFFICE TUNING CONTROL SYSTEM Application October 12, 1945, Serial No. 622,047

1 Claim. 1

My invention relates to control systems and, in particular, relates to automatic control systems for maintaining desirable conditions on high frequency heating systems and the like. Certain subject matter herein disclosed is being claimed in my copending application Serial No. 113,153, filed August 30, 1949, for Control Systems.

In recent years, systems comprising high frequency electric generators of a type well-known in the radio art have come into growing use for heating various substances, either by eddy currents induced therein by high frequency electric fields, or by the dielectric losses occurring therein when subjected to the influence of high frequency electric fields. In order to utilize such generators and their connected circuits to the greatest efficiency and otherwise most advantageously, it is usually necessary to have their circuit parameters and constants adjusted to suit the electric and other properties of the load to be heated; for example, it is frequently advantageous to have a circuit of which the load constitutes one element tuned to resonance with the resonant circuit comprising part of an electronic generator or other source of the high frequency power. It is a simple matter to adjust the constants of the generator to suit the electric circuit embodying the load to be heated initially; but as the temperature of the heated substance rises, its dielectric constant, electrical resistance, permeability or other electrical property is likely to undergo change with temperature, thereby altering the values to which the generator or its circuit should be adjusted. For example, alteration of the dielectric constant of the load may change the frequency at which the load circuit naturally resonates.

One object of my invention is, accordingly, to provide an arrangement for automatically adjusting the constants of a high frequency elec-, trical generator or a heating circuit it supplies to maintain a, desired relationship between them.

Another object of my invention is .to provide an arrangement for automatically regulating the resonant frequency of a high frequency heating circuit and an electric generator supplying current thereto, to maintain desired relationship between them at all times during operation.

Another object of my invention is to provide an arrangement for automaticall regulating the resonant frequency of a high frequency heating circuit and an electric generator supplying current thereto, to maintain substantial equality betweenthem at all times during operation.

Still. another object of my. invention is to provide an automatic system for maintaining the circuit constants of a high frequency load circuit and an electric generator supplying current thereto in such relation to each other as to maintain a substantially constant current in the load circuit.

Still another object of my invention is to provide a system for regulating the direct-current voltage supplied to a high frequency electric generator in such a way as to maintain current of a predetermined magnitude in heating load circuit supplied with current from said generator.

The foregoing and other objects of my invention Will become apparent upon reading the following description taken in connection with the drawing, in which:

Figure 1 is a schematic diagram of a high frequency electronic generator supplying current to a dielectric load for heating purposes and provided with regulating means for maintaining a desired relationship, for instance, equality, of resonant frequency between the resonant circuit of the generator and the load circuit in accordance with one embodiment of my invention;

Fig. 2 is a schematic diagram of a modification of the invention of Fig. 1, useful when the load to be heated in a conducting body heated primarily by a high frequency electric current induced therein;

Fig. 3 is a schematic showing of a modification of my invention in which an electronic generator supplying high frequency current to heat a dielectric load is regulated to maintain constant current flow through the load regardless of variations in the electrical and dielectric constants thereof; and

Fig. 4 is a schematic diagram of a modification of the circuit of Fig. 3 in which constancy of current flow in a dielectric load is maintained by varying the direct-current voltage supplied to the electronic generator.

Referring in detail to Figure 1, an electronic oscillation generator I of any suitable type wellknown in the art has an anode 2 connected through a suitable choke coil 3 to the positive terminal of a direct-current source of which the negative terminal is grounded. The anode 2 is likewise connected to ground through a variable capacitor 4 which is shunted by an inductor 5 in series with a second capacitor 6. An intermediate point on the inductor 5 is connected to the oathode l of the tube I and the control electrode 8 of the tube I is connected to ground through a 1 suitable resistor 9 shunted by th capacitor [0.

I have chosen to illustrate theoscillation generator l as connected to a heating circuit for a load ll through a transmission line S2 of any suitable length and form well-known in the art. Power is fed from the inductor to the transmission line 12 through a winding [3 inductively related to the inductor ii, and power is fed from the outgoing end of th line I2 to the load ciruit through a primary Winding I4 which is preferably in rather loosely-coupled relationship with a secondary winding It. The load II, which may comprise a pair of end plates l6, ll, having placed between them any dielectric material which it is desired to heat, is connected directly across the terminals of thewinding l5, which terminals are likewise shunted by a variable capacitor 18.

In order to efiiciently use the above-described system for heating the load H, it is usually desirable to have the resonant frequency of the circuit comprising the winding l5 shunted by the dielectric of theload H and the capacitor l8 tuned to resonate at the same frequency as the tank circuit ll, 5, 6 of the generator l. While it is easy to adjust the capacitor I8 or winding to bring about such resonance at the start of the heating cycle for the load II, it is generally found that the dielectric constant of the load II will change a its temperature rises, thereby disturbing the tuning of the resonant circuit ll, i5, 13. In order to provide an apparatus to automatically adjust the capacitor 3 or the actual capacity of load H to maintain resonant tuning between the circuit II, It, I8 and the tank circuit l, 5, G, I provide a pair of rectifiers l3, 2|, of any suitable type having similar electrodes connected to two tap points on the Winding IS. The other terminals of the rectifiers l9 and 2| are respectively connected to each other through a pair of capacitors 22, 23 and a pair of resistors 26, 25. The intermediate terminals of the capacitors 22, 23 and resistors 24, 25 are connected together, and likewise connected through an impedance 26 to a point on the winding I5 halfway between the taps to which the rectifiers l8 and El are connected. A capacitor 27 is connected between a midpoint of the winding l4 and the last-mentioned tap-point.

The arrangement comprising the elements l9 through 27 constitutes an electric circuit which can be shown to have Zero direct-current voltage between the opposite terminals of the resistors 24, 25 when the resonant frequency of the circuit ii, i5, i8 equals the resonant frequency of the tank circuit 6, 5, 6. There will be found between 'the opposite terminals of the resistors 24 and 25 a direct-current voltage which will depend in polarity upon whether the resonant frequency of the circuit comprising elements Ii, 15 and l3=is 'below that of the resonant frequency of the tank circuit 4, 5, 6 or is above the resonant frequency of the latter. In other words, aslong as the resonant frequency of the circuit H, l5, it is the same as that of the tank circuit 1i, 5, 5, the voltage difference between the outer ends of the resistors 24, 25 will be zero; but, if the resonant frequency of the circuit H, l5, it falls below that of the circuit 11,5, 6, a direct-current potential'd-ifierence will appear between the opposite terminals of the resistors .24., 25 which, over a wide range of frequency difference, will be proportional to'that frequency difierence. On the other hand,if the resonant frequency of the circuit H, l5, l8 rises abovethat of the circuit 4, 5, 6, a direct-current voltage will appear across the opposite terminals 'of' the resistors 26, '25, r

anodes of the tubes which is of the reversed polarity to that just described and which will likewise be proportional to the difference in frequencies between the two circuits over a wide range thereof.

By mechanically connecting the adjusting element of the capacitor It or winding 55 or load If to a suitable electric motor, of which the direction of movement is dependent upon the polarity of the voltage difference between the opposed terminals of the resistors 24 and 25, it

'is possible to cause such a motor to move the variable element of the capacitor l8 until it reestablishes equality of resonant frequency between the circuit ll, l5, l8 and the circuit 4, 5,

"5. One way in which such a motor may be operated is to providea pair of electronic triodes 23,

29 having theircathodes connected together to 'the negative'terminal of a suitable direct-curto the common terminal of the resistors 24, 25.

The windings 32,33 and the tubes 28, 29 are so adjusted that, when there is no difference of direct-current potential between the opposite ends of the resistors 24, 25, the current flowing in the tubes zit-and 29 exert equal and opposite pulls on the core 34.

With the arrangement just described, the appearance of a direct-current potential difference between the opposed terminals of the resistors 2d, 25, indicative as above described of a departure from resonance of the circuit ll, l5, I8 from equality with the resonant frequency of the circuit 4, 5, 6 will cause an increased current flow through one of the tubes'28 and 29 and a decreased current fiow through the other, thereby displacing the core 34. This displacement of the core 3 1 is arranged to move the variable element of capacitor It in theproper direction to bring back equality between the resonant frequencies of the circuit ii, l5, l3 and the circuit l, 5, B. Whensuch equality ofresonant frequences is thus again established, the current flow in the tubes 28 and-29 will again be equal and no further movement'of the core 34 ,will take place-until such equality -.again ceases to exist.

While I have shown the motorgarmature 34 as attached to move the variablemember of the capacitor l8, it is obvious that it may instead be attached-to move the variable member of the capacitor-4 controllingthe tuning of the tank circuit of the generator I #In such a case, the

generator I' would be continually varied in frecore 34 the arrangement may be made to maintain anypredetermineddegree of departure from exact resonance inaccordance" with-principles well-knowninthe art. In-a broader aspect my concept is that the discriminator 1'9, 26 may maintain any predetermined adjustment of the constants or parameters of the generator and load circuits. For example, the spacing of electrodes holding a dielectric load may be adjusted; or the inductance or capacity of the generator or work circuit; or the coupling between the generator and the work circuit; or the magnitude of the plate or grid voltages; or the wave form of pulsating voltages impressed on the plate or con-v trol electrodes of the generator as described in application Serial Number 542,981 of J. R. Bykin, filed June 30, 1944, and assigned to the assignee of this disclosure.

Fig. 1 thus shows an arrangement for maintaining equality of tuning between a dielectric heating load circuit and the tank circuit of an electronic generator supplying current thereto. However, it is frequently desirable to maintain similar resonance between different portions of the circuit of an electronic generator supplying energy to heat a metallic or other load by currents induced therein through magnetic action of a coil traversed by high frequency current and Fig. 2 shows a modification of my invention adapted to such a purpose.

Since, in the Fig. 2 arrangement, the main circuits of the high frequency generator may be the same as those illustrated in Fig. 1, I have confined the diagram of Fig. 2 to the tank circuit of the oscillation generator and the induction heating load fed thereby. Thus, the tank circuit of the generator may comprise a capacitor 4 similar to the capacitor 4 in Fig. 1, and likewise an inductor 4| in series with a second capacitor 42. The capacitor 42 is shunted by a circuit comprising an inductor 43 in series with the inducing winding 44 of a high frequency electric furnace. The winding 44 surrounds, and is adapted to induce eddy currents in, a suitable load 45 which is to be heated, and which may, for example, comprise a continuously moving strip. The midpoint of the winding 44 is conheated. through a suitable capacitor 46 to a tappoint on the inductor 4 To the end terminals of the winding 44 are connected a pair of rectifiers 41, 48 analogous to the rectifiers I9, 2| of Fig. 1. Across the free terminals of the rectifiers 41, 48 are connected two serially connected identical condensers 22,

23, and two serially connected identical resistors 24, 25. The common terminals of the resistors 24, 25 and capacitors 22, 23 are connected together to one end of an impedance 26 of which the opposite end is connected to the midtap of the winding 44.

As in the case of the resistors 24, 25 of Fig. 1, the similarly numbered resistors in Fig. 2 will show, across their opposite terminals, a directcurrent voltage which will be zero when the local circuit42, 43, 44 resonates at the same frequency as the tank circuit 4, 4|, 42. Similarly, the direct-current voltage between the opposite ter- .minals of the resistors 24, 25 will be proportional to, and of one polarity, when the resonant frequency of the local circuit 42, 43, 44 is less than that of the tank circuit 4, 4|, 42, and of the opposite polarity when the resonant frequency of the local circuit 42, 43, 44 is greater than that of the local circuit 44, 4|, 42. As in the case of the Fig. 1 arrangement, the voltage across the outside terminals of the resistors 24, 25 may be used to control a motor regulating the frequency determining elements of either oscillator or load circuit 42, 43, 44 to maintain the resonant frequency of the circuit 4, 4|, 42 equal at all times to that of the local circuit 42, 43, 44 in spite of any variations which the latter may undergo by reason of changes in the electrical constants of the winding 44 and the heated load 45 handled thereby. The coupling between the load and oscillator may likewise be controlled by such a motor; for instance, by varying capacie tor 42 or inductor 4|. In general, thevoltage across resistors 24, 25 may make the adjustments discussed above in describing Fig. 1. Fig. 3 shows another embodiment of my in vention which I illustrate in connection with a high frequency electric generator used to heat a dielectric load analogous to the load II de-. scribed in connection with Fig. 1. The generator tube I and the elements connected therewith numbered 2 through It are identical with those already described in Fig. 1 and need not be alluded to in detail here. However, there is provided, in inductive relation with the inductor 5 and connected between the end of the latter and the ground, a reactance 5| which may, for example, be a small inductor. The reactance5| is shunted by a circuit comprising a rectifier 52 inseries with a resistor 53. 11-

I have shown the dielectric load connected to the inductor 5, not by a transmission line such as the line l2 in Fig. 1, but by a variable inductor 54 and a pair of variable capacitors- 55, .55. The variable element of one of the capacitors 55, 56 is connectedin a manner similar to that already described for the variable element ofv capacitor H5 in Fig. 1, to the core 34 of a reciprocating motor having a pair of suitable windings 51 and 58. The midpoint of the windings 51, lstconnected to the positive terminal of a direct-current voltagesource 59 of which the negative. ten.- minal is grounded. The free terminal ofs the winding55 isconnected to the negative terminal voltage source 59 through a variable resistor 5|. The free terminal of the winding 51' is connected to the anode of a triode 62 the cathode of which is grounded. The control electrode of the tube '52 is connected to a tap-pointon the resistor 53. .At the initiation of a heating cycle, the cur rent through the load H, which will readily be seen to likewise flow through the reactance 5|, is adjusted to a desired value at which it is intended .to be maintained by moving the variable ele-v ment of the capacitor 55. At the same time, the resistor 6 is adjusted so that the magnetic efiect or" the windings 5'! and 58 on the core 34 exactly balanceeach other. If, thereafter, the resonant frequency of the circuit containing the load II, the capacitors 55, 56, the inductors 54, 5 and 5|, orany of its other properties, undergoes a; change as heating proceeds so that the current flow through the reactor varies from the above-mentioned value at whichit has been initially adjusted, the direct-current voltage drop across resister 53 will vary, thereby impressing a different bias voltage" on the control electrode of the tube 52. This will cause a variation in the current through the winding 51 with the result that the core 34 will move in such a direction as to adjust the capacitor 55. Such an adjusting movement will continue until the current through the reactor 5| returns again to its original value, "at which time the bias voltage impressed by resistor 53 on tube 52 will return to its original value. In consequence of such return, the current through the winding 51 will again balance the pull of the winding 58 on core 34 and further movement of the variable capacitor 56 will cease.

It Will be evident that by varying the bias 7 acting on core .in .any. -predetermined:.desired waythe'load current may-be made to followaany desired program or relationship to the biascontrolling: :quantity.

-W hile' Izhave described :in Fig." 3: a modification of :my: invention in which the adjustment r for constantuourrent fiow in the heating load .is brought about byvariation of coupling .01 other elements. in the heating circuit,-it is likewise possible to employ =-the direct-current. output voltage dropacross the resistor53 to regulate for constant load currentby altering the directcurrent voltage impressed upon the-plate :oi the oscillator I. Fig. 4-shows details of the portion of the circuit in which thisdirect-ourrent voltage regulation is accomplished. iThus, .voltage' for theplata of theoscillationagenerator 2 .isobtainable from a pair of grid-controlled rectifier tubes- 1 I 12 having their :anod-es fed: from the opposite terminals -of the secondary winding l3 of a conventional.supplytransformer. The cathodes of therectifiers ll, 12 are connected directly through the choke coils to theanodel .of theoscillator land-themidpoint of the'winding 13 is connected to ground. Between the cathodes of the-tubes ll, 12 and their control electrodes are connected the secondary windings 14 of a peaking transformer shunted respectively by R. F. bypass capacitors 15. Energizing the secondary windings 14 is aprimary winding 16 fed from-an alternating-voltage source through a saturable reactor winding H. The magnetic saturation of an iron core for winding 11 is controlled by-the plate currentofa triode 18 having its grid voltage fixed by the drop across the resistor 53 of Fig. 3. The'saturable reactor system just-described is well-known in the electronic art for controlling current= flow in grid-controlled tubes such as rectifiers Hand-12. Whenthe Fig. 4 arrangementis-ernployed, the elements56, 51,58,59, BI, 62 and 34, may, of course, be-ornitted from Fig. 3.

-'At the initiation of the heating cycle, the current through the load ll is "adjusted-toits-desired'value by'variation of the variable capacitor 55'. If, at-any time thereafter, the I current through the load-should rise above the desired value,- the direct-current voltage drop between the tap or resistor 53 and ground will obviously increase, thereby increasingcurrent flow through the tubel8,-and so altering the condition of reactor ll that the pulses from windings 14 act through-the grids to render'tubes H and 12 conductive lateri-in the alternating-current cycle. As a result, the voltage impressed on the anode of the oscillator I will. decrease and the latter will-furnish. lesspower to the load ll. .As a result, the current flow through the. load [I will fallzbaclcagain to. the initial. value at which. it was desired toxmaintain it. "Itis obvious that should-the. currentfiow. through. the load. I I .decreasathe voltage. drop through resistor 53 will similarly decrease with the result that a more positive voltage on the grids of rectifiers H and 12 will cause the. impression of increased voltage on the anodes. of theoscillator I. The latter. will thus react to return current flow through the load tothe value of its initial adjustment.

.Byconnecting the. rectifier52 and. resistor 53 across suitable tap-points. on an inductor in an induction-heating circuit, it is believed obvious, without further explanation, how the methods OfJFig. '3'aandFig- 4 may be employed to maintain, constant current flow through .an induction heatingloadin a manner analogous to that in which, they are applied Figs. 3.and 4 to maintain. constantncurrent flow through a dielectric load 1 such .as H. Alternatively by connecting rectifier 52 and resistor 53 across a tap on the inducing-winding on an induction heater, constant voltagermaybe maintained on. said inducing winding.

It is likewise believed to be obvious that .the cores 34 in the arrangements of Fig. 1 and Fig.3 maybe used to vary such. coupling elements as thexwindings l3 and M of Fig. 1, any frequency determiningLelementsof oscillator I, resistance. 9 inFigs. 1 and 2,the positionof the connection from the cathode to inductors'5 in Figs. 1 and'3, or by'varying control voltages for induction heating, .such asare described in application Serial Numberi542,981, ofJ. R. Boykin,'filed June 30, 1944,.and. assigned to the assignee of this application. By varying the position on resistor 53 of the tap to the gridof tube 78: in any predetermined manner the voltage supplied to the generator may be: made. to. follow any-desired: program or relationship' to: the quantitycontrolling that position.

:It. is likewise believed to besobvious' that the rectiflers l9and2l and the capacitorll may respectively-be connected to thewinding I3 and theportion of. inductors adjacent thereto instead oitothe-windings l5 and. I4.

; I:clairn-.as my invention:

'I11; c0mbinati0n with an oscillation generator anchai load. circuit to. be heated with energy derived therefrom, means for loosely coupling said load'oircuit to said generator, means including a reactance for-imparting to said load circuit a natural frequency, a pair. of rectifiers havinginputrterminals connected to two points spaced apart onsaid; reactance, a pair-of resistors connested in series between the output terminals of said 'rectifiers, a connection between the midpoint of said resistors and awpcint intermediate between the twopoints above-mentioned on said reactance, and means responsive to variations in magnitude of thedirect-current potential difference [between the opposite. terminals of. said resistorsfor adjusting said natural frequency.

THEODORE P. KINN.

REF.ERFIFK'DES CITED "The followingreferences are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date :2g044fl49. Usselman June 16, 1936 2,147,689 'Chafiee Feb. 21, 1939 925231g4'5'7 Stephen ;Feb. 11, 1941 212513277 Hart'et a1 -Aug.- 5, 1941 2,324,525 Mittelmann July 20, 1943 2,358,454 Goldstine Sept. 19, 1944 "2,387,544 Usselman Oct; 23, 1945 2 ,396,004 Gilbert Mar. 5, 1946 2,415,799 Reifel et al Feb. 11, 1947 2,416,172 Gregory-ct a1 "Feb. 18, 1947 

